Existing successful heat engines are steam turbines, gas turbines and positive displacement engines (reciprocating piston and rotary Wankel) utilizing various thermodynamic cycles (Diesel (or rather Sabathe), Otto and Stirling cycle). These engines, although now having been developed for more than century (almost 2 centuries in the case of Stirling), still stop short from fulfilling the requirements imposed on prime movers by modern economy. Thus steam turbines require huge steam boilers and steam condensers and are troublesome to exploit, therefore their applications are restricted to power plants and propulsion of ships and some other heavy machinery. Moreover the overall efficiency of steam turbine power plants is significantly inferior to that of large Diesel engines. Most important advantage of steam turbines over other known heat engines is their ability to cope with large outputs, approximately 2 000 MW in largest units, i.e. 25 times the output of largest reciprocating engines. Another important advantage of power plants utilizing steam turbines is their multi-fuel capability.
Gas turbines, thermal efficiency of which can achieve even 55% in large units destined for power generation and industrial applications (which in fact are compound heat machines with large heat exchanger), usually, particularly in small units, display much poorer figure than positive displacement engines, are more complicated technologically and more expensive, and therefore are unlikely to earn as dominant position as Diesels enjoy today due to these and other well-known inherent drawbacks and limitations. Moreover, gas turbines require high quality, expensive fuels, and their ability to accept cheap solid particulate fuels (mainly pulverized coal) is problematic, due to unacceptably low durability of turbine's blades coming in contact with unburned hard solid particulates produced by burning pulverized coal.
Thus positive displacement engines still have important advantages over turbines that render them irreplaceable for most applications.
Most common positive displacement engine in use (and in fact most common heat engine), Diesel engine, achieves maximum overall efficiency of slightly beyond 50% (large stationary or marine units, which again are compound heat machines comprising Diesel engine, turbocharger, supercharging air cooler and auxiliary power turbine), and average Diesel efficiency is merely ˜40%, a poor figure in comparison with 70-75% originally assumed by its inventor in late 19th century. Thermal efficiency of Diesel cycle rises with the compression ratio, but this method for improving overall efficiency of real Diesel engines is negated by friction loses generated by various engine's kinetic couples loaded with the gas force (piston-connecting rod, crosshead-slider, connecting rod-crankshaft, crankshaft-engine's body) rapidly rising with loads of engine's mechanism. Moreover, conventional connecting rod—crank mechanism's strength becomes a concern in highly loaded Diesel engines.
Another well-known positive displacement heat engine is the (external combustion) Stirling engine. This engine is closest to the ideal Carnot engine in terms of thermal efficiency, and another important advantage over known internal combustion engines is its capability to utilize various sources of thermal energy. However, Stirling engine is expensive to manufacture and troublesome to maintain, and this renders it considerably inferior to internal combustion engine in most applications, and prevents from earning wide acceptance. Moreover, Stirling engine shares with diesels main disadvantages of reciprocating engines mentioned above.
There are many non-conventional designs of heat engines (most of them focusing on transforming gas force into driving torque of rotating shaft), e.g. rotary engines like Wankel, recently patented quasi turbine (see U.S. Pat. Nos. 6,164,263 and 6,899,075), spherical engines (see U.S. Pat. Nos. 6,325,038, and 6,941,900, and Russian patent 2,227,211) and oscillating pivotal engine (see www.PivotalEngine.com). However, so far none of those non-conventional engines, with Wankel-type engine being the only exception of economically (but certainly not conceptually) marginal importance, was successful, and probably none of them includes any chance to even go beyond the stage of prototyping. Technically, this is due to the fact that the answer to the principal question any new engine is obliged to answer: “Does the new engine do its work better than conventional one?” is decidedly negative for all those non-conventional designs, including Wankel's. Even the answer to the more general question: “Does the new engine do its work in any aspect better than conventional one?” is negative for almost all non-conventional engines. (In the case of the Wankel engine, the answer to this more general question is positive, but superiority of Wankel over conventional engines in certain aspects (great power/weight and power/volume ratios, kinetic simplicity and smoothness of operation) is overshadowed by its inherent drawbacks (weak structure, inability to cope with large outputs, inferior efficiency, weakness of sealing, inherent inability to incorporate high compression ratios and to cope with large outputs)). Conceptually, this is mainly due to the fact that those new engine designs (e.g. quasi turbine) focus on certain isolated aspects of heat engine while ignoring some other aspects (e.g. sealing, mechanical strength and reliability).
For example, recently patented positive displacement rotary engine, quasi-turbine, is complex both kinetically and structurally, its moving elements of complicated shapes are likely to be subjected to excessive thermal stresses and renders the engine weak structurally and more difficult to seal than Wankel engine; thus the engine is unlikely to do well the job of heat engine (it would be better as pump or compressor). Some other rotary engines (e.g. satellite engine, see publication WO9618024) use toothed wheels to transfer the pistons movement to rotary motion of engine's shaft. This not only makes these engines complex but also unreliable, as engine's elements that meet along a line are not well suited to bear shock loads met with in internal combustion engines.
Fuel cell is a very promising source of power for many applications, but it seems improbable it will become appropriate for applications where high power density and large power are essential in any foreseeable future.
Electricity is most often generated by electromechanical generators, primarily driven by heat engines fueled by combustible fuels like coal dust, or nuclear fission, but also by other means such as water turbines utilizing kinetic energy of flowing water. Capability of achieving large powers (250-2000 MW), revolving very fast (3000-3600 rev/min) to be able to directly (i.e. without the necessity of being augmented by large multiplication gears, which would be impractical if not impossible) drive generators, and capability of being fueled by cheap solid fuels like coal dust, are well known to be essential requirements for heat engines destined for applications in power plants. By now only steam turbines or closed cycle turbines (e.g. utilizing helium as a motive fluid) are capable of fulfilling these requirements. Contemporary reciprocating internal combustion engines are well known to hardly accept fueling by coal dust due to unacceptably rapid wear of engine's moving parts caused by unburned solid hard particulates produced by combustion of such fuel (some attempts have been made to fuel reciprocating internal combustion engines by coal dust (see for example U.S. Pat. Nos. 4,052,963, 4,056,080, 4,070,996, 4,070,997, 4,086,883, all assigned to Sulzer Bros. Ltd.), but, up to my knowledge, none of these attempts was a success; to be more specific, the problem of fueling Diesel engine by coal dust is only partially solved: the problem of supplying coal dust to engine's cylinder in precisely metered portions seems to be satisfactorily solved, while removing unburned solid particulates from engine's cylinder is still a serious challenge).
Moreover maximum power achieved by reciprocating engines is approximately 80-87 MW (Wartsila-Sulzer RTA96-C or largest MAN-B&W 14-cylinder engines weighing ˜2300 metric tons), and largest reciprocating engines are not capable to revolve faster than approximately 100 rev/min due to large inertia mass forces generated by heavy reciprocating engine's parts (piston, piston rod, crosshead, and connecting rod, total weight of which amounts to tens of tons multiplied by the number of engine's cylinders), poor figures in comparison with steam turbines; all these drawbacks render reciprocating internal combustion engines impractical as far as large scale power generation is concerned. Note, that increasing rotational speed of the RTA96 engine mentioned above to 3000 rev/min would increase its output to more than 2500 MW, a goal that obviously cannot be achieved due to at least 30-fold increase of mass forces that would inevitably have destroyed the engine (of course there are also other well known obstacles).
Thus there is a need for highly efficient internal combustion engine capable of being fueled by coal dust (which is the cheapest combustible fuel), and capable of developing very large power comparable to largest steam turbines, while revolving at 3000-3600 revolutions per minute.
It is to be stressed that none of the non-conventional engine designs in United States Patent and Trademark Office (USPTO) and European Patent Office (EPO) patent data bases offers satisfactory mechanical structure of the ICE suitable for coping with extreme large power and high rotary speed while assuring engine's compactness and good sealing. Moreover, none of the known positive-displacement internal combustion engines approaches highly desirable kinetic simplicity of gas turbines.